The present invention relates to the production of high octane number gasoline using a process combining at least one hydroisomerisation section and at least one section for separation by adsorption in which the adsorbent is a microporous solid zeolite with a mixed structure, with channels with distinct sizes.
More precisely, the process of the invention can produce a high octane number gasoline stock that forms part of the composition of the gasoline pool.
The quality of a gasoline is partially dependent on its octane number. From the point of view of the octane number, the hydrocarbons constituting the gasoline are preferably as highly branched as possible as shown by the research octane number (RON) and motor octane number (MON) of different hydrocarbon compounds (see table below).
A number of techniques have been proposed in order to increase the octane number of a gasoline. Firstly, aromatic compounds, principal constituents of reforming gasolines, and isoparaffins produced by aliphatic alkylation or isomerisation of light gasolines have compensated for the loss of octane number resulting from removing lead from gasolines, this removal being as a result of environmental constraints that are constantly being tightened up. As a result, oxygen-containing compounds such as methyl tertiobutyl ether (MTBE) or ethyl tertiobutyl ether (ETBE) have been introduced into the fuels. More recently, the acknowledged toxicity of compounds such as aromatics, in particular benzene, olefins and sulphur-containing compounds, and the desire to reduce the vapour pressure of gasolines, have resulted in the production of reformulated gasoline. As an example, since Jan. 1, 2000, the maximum olefin content, total aromatics content and benzene content of gasoline distributed in France are respectively 18% by volume, 42% by volume and 1% by volume.
Gasoline pools comprise a number of components. The major components are reforming gasoline, which normally comprises 60% to 80% by volume of aromatic compounds, and FCC gasolines, which typically contain 35% by volume of aromatics but supply the majority of the olefinic and sulphur-containing compounds present in the gasoline pool. The other components can be alkylates, with neither aromatic compounds nor olefins, light isomerised or non isomerised gasolines, which contain no unsaturated compounds, oxygen-containing compounds such as MTBE, and butanes. Provided that the aromatics content is not reduced to below 35-40 vol %, the contribution of reformates to gasoline pools remains high, typically 40 vol %. In contrast, increased tightening of the maximum admissible aromatic compounds content to 20-25 vol % will cause a reduction in the use of reforming, and as a result will need straight run C7-C10 cuts to be upgraded by methods other than reforming.
To this end, the production of multibranched isomers from slightly branched heptanes and octanes contained in naphthas, instead of producing toluene and xylenes from those compounds, appears to be a promising route. This justifies the search for catalysts that perform well for heptane isomerisation (also known as hydroisomerisation when carried out in the presence of hydrogen), octanes and, more generally, C5-C8 cuts and intermediate cuts, and the search for processes that can selectively recycle the low octane number compounds, namely linear and monobranched paraffins to the isomerisation (hydroisomerisation) stage.
In order to selectively recycle the linear and monobranched paraffins to the hydroisomerisation stage and to recover multibranched paraffins with a high octane number, to introduce them into the composition of the gasoline pool, the multibranched paraffins must be separated at least once. A separation unit, producing at least two distinct effluents, one with a high octane number and the other with a low octane number, and integrated into a process also comprising at least one hydroisomerisation unit to recycle the low octane number effluent to the hydroisomerisation unit, which converts linear paraffins and monobranched paraffins with a low octane number to multibranched paraffins with a high octane number.
The main difficulty in carrying out such a process combining hydroisomerisation and separation steps is separating the multibranched paraffins.
Techniques for separation by adsorption, using molecular sieves that are selective because of the dimensions of their accessible pores, are particularly suitable for separating linear, monobranched and multibranched paraffins. Conventional adsorption processes can result in procedures of the PSA (pressure swing adsorption), TSA (temperature swing adsorption), chromatographic (elution chromatography or simulated counter-current) type, for example. They can also result in a combination of these procedures. Such processes all bring a liquid or gaseous mixture into contact with a fixed bed of adsorbent to eliminate certain constituents of the mixture that may be adsorbed. Desorption can be carried out by different means. The common characteristic of PSA is that the bed is regenerated by de-pressurisation and in some cases by a low pressure flush. PSA type processes have been described in U.S. Pat. No. 3,430,418 or in the more general work by Yang (xe2x80x9cGas separation by adsorption processesxe2x80x9d, Butterworth Publishers, US, 1987) In general, PSA type processes are operated sequentially and using all the adsorption beds in alternation. Such PSA processes have been successful in the natural gas field, for separating the compounds in air, for producing solvent and in different refining sectors.
TSA processes use temperature as the desorption driving force and were the first adsorption processes to be developed. The bed to be regenerated is heated by circulating a preheated gas in an open or closed loop in the reverse direction to that of the adsorption step. A number of variations of the schemes (xe2x80x9cGas separation by adsorption processesxe2x80x9d, Butterworth Publishers, US, 1987) are used depending on local constraints and on the nature of the gas employed. This technique is generally used in purification processes (drying, gas and liquid desulphurisation, natural gas purification: U.S. Pat. No. 4,770,676).
Gas or liquid phase chromatography is a highly effective separation technique because of the very large number of theoretical plates (Belgian patent BE 891 522, Seko M., Miyake J., Inada K.; Ind. Eng. Chem. Prod. Res. Develop., 1979, 18, 263). It means that relatively low adsorption selectivities can be employed and difficult separations can be carried out. The competition from simulated moving bed or simulated counter-current processes for these processes is stiff. These latter processes have been developed to a great extent in the petroleum industry (U.S. Pat. No. 3,636,121, U.S. Pat. No. 3,997,620 and U.S. Pat. No. 6,069,289). The adsorbent is regenerated using the technique for displacement by a desorbent, which can optionally be separated by distillation of the extract and raffinate.
Linear, monobranched and multibranched paraffins can be separated by adsorption by different techniques that are well known to the skilled person: separation by thermodynamic adsorption difference, and separation by differences in the adsorption kinetics of the species to be separated. Depending on the technique used, the selected adsorbent will have different pore diameters. Zeolites, composed of channels, are the adsorbents of choice to separate such paraffins.
The term xe2x80x9cpore diameterxe2x80x9d is known to the skilled person. It is used as a functional definition of pore size in terms of the size of the molecule that can enter into the pore. It does not define the actual dimension of the pore as that is often difficult to determine, since it often has an irregular shape (i.e., non circular). D. W. Breck provides a discussion on effective pore diameter in the book entitled xe2x80x9cZeolite molecular sieves (John Wiley and Sons, New York, 1974) on pages 633 to 641. The cross sections of the zeolite channels are rings of oxygen atoms, so the zeolite pore size can also be defined by the number of oxygen atoms forming the annular cross section of the rings, termed xe2x80x9cmember ringsxe2x80x9d, MR. This is shown, for example, in xe2x80x9cThe atlas of zeolite structure typesxe2x80x9d, W. M. Meier and D. H. Olson, 4th edition, 1996), which indicates that FAU structure type zeolites have a crystal channel network of 12 MR, i.e., the cross section is constituted by 12 oxygen atoms. This definition is well known to the skilled person and will be used below.
The use of adsorption separation processes to fractionate feeds containing linear, monobranched and multibranched plasmids is well known and many patents make reference thereto. Different adsorbents have been recognised in those patents.
In the case of xe2x80x9cthermodynamicxe2x80x9d separation, the adsorbent has a pore diameter that is higher than the critical diameter of the molecules to be separated. A number of patents describe the separation of multibranched paraffins from linear and monobranched paraffins by selective thermodynamic adsorption. U.S. Pat. No. 5,107,052 proposes preferably adsorption of multibranched paraffins on SAPO-5, AIPO-5, SSZ-24, MgAPO-5 or MAPSO-5 zeolites. U.S. Pat. No. 3,706,813 proposes the same type of selectivity on barium-exchanged X or Y zeolites. U.S. Pat. No. 6,069,289, on the other hand, proposes the use of zeolites with selectivities that are inversely proportional to the degree of branching of the paraffins, such as beta, X or Y zeolites exchanged with alkali or alkaline earth cations, SAPO-31, MAPO-31 zeolites. All of the zeolites cited above have pore diameters of 12 MR.
In the case of xe2x80x9cdiffusionalxe2x80x9d separation, the separating power of the adsorbent is due to the difference in the diffusion kinetics of the molecules to be separated in the zeolite pores. In the case of separation of multibranched paraffins from monobranched and linear paraffins, the fact that the higher the degree of branching, the higher the kinetic diameter of the molecule, and thus the slower the diffusion kinetics, can be exploited. For the adsorbent to have a separating power, the adsorbent must have a pore diameter close to that of the molecules to be separated, which corresponds to zeolites with a pore diameter of 10 MR. Many patents describe the separation of linear, monobranched and multibranched paraffins by diffusional selectivity. U.S. Pat. Nos. 4,717,784, 4,804,802, 4,855,529 and 4,982,048 use adsorbents with channel sizes between 8 and 10 MR, the preferred adsorbent being ferrierite. U.S. Pat. No. 4,982,052 recommends the use of silicalite. U.S. Pat. Nos. 4,956,521, 5,055,633 and 5,055,634 describe the use of zeolites with elliptical cross section pores with dimensions in the range 5.0 to 5.5 xc3x85 along the minor axis and about 5.5 to 6.0 xc3x85 along the major axis, in particular ZSM-5 and its dealuminated form, or silicalite or with dimensions in the range 4.5 to 5.0 xc3x85, in particular ferrierite, ZSM-23 and XZSM-11.
The zeolitic adsorbents proposed for diffusional separation of multibranched paraffins have a homogeneous channel size structure and are only composed of small channels (8 to 10 MR), which considerably reduces their adsorption capacity. Such materials, which suffer primarily from their low adsorption capacity, cannot produce optimum efficiency of the separation unit. The performance of a process combining both hydroisomerisation and separation by adsorption would, therefore, inevitably be hampered.
The present invention is based on the novel use of zeolitic adsorbents with a mixed structure, composed of two channel types with distinct sizes, in a section for separating multibranched paraffins comprised in a hydrocarbon feed constituted by a cut in the range C5 to C8 and containing linear, monobranched and multibranched paraffins, said separation section being integrated into a process also comprising at least one hydroisomerisation section. The process of the invention comprises at least one hydroisomerisation section and at least one section for separating multibranched paraffins functioning by adsorption and containing at least one zeolitic adsorbent with a mixed structure with principal channels with an opening defined by a ring of 10 oxygen atoms (also termed 10 MR) and secondary channels with an opening defined by a ring of at least 12 oxygen atoms (12 MR), the channels of at least 12 MR only being accessible to the feed to be separated via the 10 MR channels.
The zeolitic adsorbents of the invention are zeolites that advantageously have structure types EUO, NES and MWW. NU-85 and NU-86 zeolites are also particularly suitable for carrying out the process of the invention.
In a first version of the process of the invention, the process comprises at least one hydroisomerisation section and at least one separation section. The hydroisomerisation section comprises at least one reactor. The separation section (composed of one or more units) produces two fluxes, a first flux that is rich in di- and tri-branched paraffins, possibly in naphthenes and aromatics, which constitutes the high octane number gasoline stock and which is sent to the gasoline pool, and a second flux that is rich in linear and monobranched paraffins that is recycled to the inlet to the hydroisomerisation section.
In a further version of the process of the invention, the overall process comprises at least two hydroisomerisation sections and at least one separation section. The separation section (composed of one or more units) produces three fluxes, a first flux that is rich in di- and tri-branched paraffins and possibly naphthenes and aromatic compounds, which constitutes a high octane number gasoline stock and which is sent to the gasoline pool, a second flux that is rich in linear paraffins that is recycled to the inlet to the first hydroisomerisation section, and a third flux that is rich in monobranched paraffins that is recycled to the inlet to the second section. Two types of implementation of this version of the process are preferred: in the first, all of the effluent from the first hydroisomerisation section traverses the second section; in the second, the effluents from the hydroisomerisation sections are sent to the separation section or sections.
The process of the invention can also produce a high octane number gasoline pool by incorporating into said pool a high octane number gasoline stock from the hydroisomerisation of cuts between C5 and C8, such as C5-C8, C5-C6, C5-C7, C6-C8, C6-C7, C8, etc.
The zeolitic adsorbents used in the separation section for implementing the process of the invention have substantially improved adsorbent properties over prior art adsorbents, in particular as regards the adsorption capacity itself. It has surprisingly been discovered that the use of a zeolitic adsorbent with at least two channel types with distinct sizes, principal channels with an opening defined by a ring of 10 oxygen atoms and secondary channels with an opening defined by a ring with at least 12 oxygen atoms, has a beneficial effect on the performance of a process for separating multibranched paraffins comprised in a hydrocarbon feed constituted by a C5 to C8 cut and containing linear, monobranched and multibranched paraffins in particular. The zeolitic adsorbent used in the separation section of the process of the invention combines good selectivity with optimum adsorption capacity, ensuring productivity gains over prior art adsorbents. This results in better yields for the process of the invention over other processes combining hydroisomerisation and separation by adsorption with prior art adsorbents.
The process of the invention leads to an improvement in the separation process combined with the hydroisomerisation process. Combining these processes upgrades light cuts comprising paraffinic, naphthenic, aromatic and olefinic hydrocarbons containing 5 to 8 carbon atoms, by hydroisomerisation and recycling low octane number paraffins, i.e., linear and monobranched paraffins, while the multibranched paraffins, with a high octane number, separated from the linear and monobranched paraffins, constitute a gasoline stock that is sent to the gasoline pool. Said base can increase the octane number of the gasoline pool.
The process of the invention aims to modify the landscape of gasoline production by reducing the aromatics content while keeping the octane number high, by sending a feed constituted by a C5-C8 cut (for example obtained from straight run distillation) or any intermediate cut between C5 and C8, not only to units for reforming and hydroisomerising C5-C6 paraffins, but to at least one hydroisomerisation section that converts linear paraffins (nCx, x=5 to 8) to branched paraffins and possibly monobranched paraffins (monoC(xxe2x88x921)) to di- and tri-branched paraffins (diCxxe2x88x922) or triC(xxe2x88x923)).